Lamp having improved press seal

ABSTRACT

A thin refractory metal ribbon is completely embedded in the press seal of a high silica glass lamp envelope. One end of the ribbon is securely held within a longitudinal slot of a lamp lead-in wire, thereby establishing physical and electrical connection between the ribbon and the lead-in wire.

United States Patent Anderson [4 1 June 6,1972

[54] LAMP HAVING IMPROVED PRESS SEAL [72} Inventor: John W. Anderson, Danvers, Mass.

[73] Assignee: Sylvania Electric Inc.

[22] Filed: Aug. 28, 1970 [21] Appl. No; 67,699

Fridrieh et al. .312/315 x 3,048,741 8/1962 Thowret ..313/315 X 3,073,985 111963 Fraser ..313/318 X 3,360,765 12/1967 Strange et a]. ..3l3/315 X FOREIGN PATENTS OR APPLlCATIONS 683,858 4/1964 Canada BIS/318' Primary Examiner-David Schonberg Assistant Examiner-Paul A. Sacher An0mey-Norman J. OMalley and James Theodosopoulos [57] ABSTRACT A thin refractory metal ribbon is completely embedded in the press seal of a high silica glass lamp envelope. One end of the ribbon is securely held within a longitudinal slot of a lamp lead-in wire, thereby establishing physical and electrical connection between the ribbon and the lead-in wire.

6 Claims, 5 Drawing figures PATENTEDJUN 6 I972 FIGJ e 3 7 2 5 IO FIG.3

FIG.4

JOHN w. ANDERSON INVENTOR GENT LAMP HAVING IIVIPROVED PRESS SEAL BACKGROUND OF THE INVENTION glass having approximately 96 percent silica) and the like.

Such lamp envelopes are usually sealed by a press seal at one or both ends thereof. The press seal is formed by heating the open end of a tubular lamp envelope to the softening point of the glass and pressingthe softened end between a pair of aws.

Examples of lamps having press seals of the type to which this invention relates are tungsten halogen lamps and the arc tubes of arc discharge lamps, such as high pressure mercury vapor lamps and metal halide lamps.

The lamp envelopes of such lamps utilize press seals in order to prevent seal failures due either to thermal expansion of the glass or of the metal conductor embedded therein. The high operating temperature of the lamps obviates the use of stem seals, such as are generally used in incandescent and fluorescent lamps, which have much lower operating temperatures;

It is known, in the art of lamp envelopes having press seals, to employ a thin refractory metal ribbon to establish electrical connection between the external lead-in wires and the internal filaments or electrodes. The use of thin metal ribbons, and the reasons therefor, are shown in U.S. Pats. Nos. 1,271,245 and 2,667,595, although the ribbons shown therein are flattened portions of a single conductor extending completely through the seal. Generally, however, the ribbon and wire are made of dissimilar metals, the ribbon metal being selected for its ductility, usually molybdenum, and the internal wire metal being selected for its refractoriness, usually tungsten.

Generally, the ribbon is connected to the lead-in wire by partial brazing or tacking. In order to make such a connection, a thin sheet of a suitable'm'etaL'usually platinum, is placed between the ribbon and the lead-in wire, and the assembly is placed between a pair of resistance welding electrodes. The assembly is pressed between the electrodes and sufiicient electric current passed therethrough to heat and tack the elements together, with at least partial melting of the platinum sheet. The bond between the ribbon and lead-in wire may be imroved during the press sealing operation if the silica glass is hot enough to melt additional platinum.

Although such a brazed or tacked connection is satisfactory for some applications, it has some disadvantages, which are alleviated by this invention.

First, the brazing or tacking process may cause recrystallization of part of the molybdenum ribbon that is directly between the resistance welding electrodes. Recrystallization results in weakening of the ribbon which can then physically rupture because of thermal cycling that occurs during normal lamp operation. Or the recrystallization can cause high resistance electrical contact between the lead-in wire and the ribbon; the heat generated by the high resistance contact causes further degradation with resultant arcing and finally failure of the press sea].

A second disadvantage is that there may be insufiicient flow of the brazing metal to insure an adequately low electrical contact resistance. Point contact, as distinguished from a desired area contact, can result in the failure mentioned above.

Still another disadvantage of the prior art seal is the possibility of reaction of the brazing metal with materials present in the lamp fill. Such a reaction is possible because the glass does not form a hermetic seal with the lead-in wires; the hermetic seal is formed only in the region of the thin ribbon. Consequently it is possible for materials in gaseous form to penetrate the minuscule passage-that exists around the lead-in wire, as far as the ribbon connection at the end of the lead-in wire.

For example, the arc tubes of metal halide lamps have a fill including light emitting metals such as are shown in US. Pat. 'Nos. 3,234,421, 3,334,261 and 3,407,327. During lamp operation, the metals are present in a gaseous state, as the halide. In the gaseous form, the metallic halides can penetrate the seal and may react with the brazing metal to form an alloy having a eutectic temperature lower than the actual operating temperature of the seal. Or else a particular light emitting metal, such as scandium iodide, could react with the glass to form silicon iodide, which, in turn, could react deleteriously with the platinum to form a platinum-silicon material. The latter material could cause seal failure in the manner mentioned above.

SUMMARY OFTl-IEINVENTION A lamp having a press seal in accordance with this invention has a refractory metalwire or rod, usually tungsten, disposed within a high silica glass enclosure which can be the lamp envelope of a high temperature incandescent lamp or the are longitudinal slot therein; securely held within the slot is one end of a thin refractory meta] ribbon, the ribbon itself being completely embedded within the press seal. The ribbon is held within the slot by clamping pressure exerted by the sides of the a slot.

In the manufacture of a press seal in accordance with this invention, a longitudinal slot is formed in one end of the refractory metal wire. The slot can be formed, for example, by grinding, machining, electrical disintegration, sonic cavitation or laser beam, although other methods may be equally suitable. Y a

The slotmust have a sufficient opening to permit insertion of the ribbon therein and must have sufiicient depth to provide adequate contact area between the sides of the slot and the ribbon. Adequate contact area is necessary in order to provide low electrical contact resistance.

After the ribbon has been inserted into the slot, the sides of the slot are physically pressed together, preferably with heat but at a temperature lower than the recystallization temperature of the metals involved. The pressureexerted must be sufficient to deform and close the slot but must not be so great as to crush or rupture the metal having the lower yield strength.

In joining metals having different yield strengths, the metal having the higher yield strength is preferably heated to a higher temperature than the other metal. Preferably, also, the heating and joining are done in an inert or reducing atmosphere, in order to prevent oxidation of the metal.

BRIEF DESCRIPTION OF DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTS In one example of an arc discharge lamp having press seals in accordance with this invention, as shown in FIG. 1, the lamp has a high silica glass envelope 1 having press seals 2 at either end thereof. Disposed within envelope 1 are electrodes 3 at either end thereof, the electrodes consisting of tungsten coils 6 mounted on tungsten rods 7. In addition, starter electrode 4 is disposed at one end of envelope 1 adjacent one electrode 3. Starter electrode 4 comprises the inwardly projecting end of a tungsten or tantalum rod 8.

Embedded within press seals 2 are thin molybdenum ribbons 5, one ribbon for each electrode. Connection between each ribbon 5 and its respective tungsten rod 7 is established, prior to formation of press seal 2, by inserting the end of ribbon 5 into a slot at the end of the rod and pressing the sides of the slot together. The other end of each ribbon can then be connected to external lead-in wires by the same method. However, prior art brazing methods may also be used in connecting the ribbon to the external lead-in wire, since the operating temperature of the seal thereat may be low enough to permit such a connection. For the same reason, brazing may be used toconnect the ribbon to the starter electrode 4.

After press seals 2 have been formed, envelope 1 is exhausted, filled and sealed by methods known to the art.

In one example of a 1,000 watt arc tube, 94 mm. long, made in accordance with this invention, each tungsten rod 7 was 45 mils in diameter. As shown in FIG. 3, slot 9 was diametrically formed at one end of rod 7, the slot being 6 mils thick and about 60 to 80 mils deep. Molybdenum ribbon 5 was rectangular, measuring about 3/16 inch by inch, and was about 1.3 mils thick at its center. A long end of ribbon 5 was centrally inserted into slot 9 and the rod was then placed between two socalled welding electrodes. The welding electrodes were made of thoriated tungsten and had a diameter at their tip of about 1/16 inch.

Rod 7 was so aligned between the welding electrodes that when the latter were closed, they exerted a substantially normal closing force on each side 10 of slot 9. A closing force of 7 pounds was applied between the welding electrodes while sufficient electric current was passed therebetween to heat the slotted portion of rod 7 to about 1,500 or l,600 C. The heat and pressure were sufficient to deform the sides of the slot and close them securely against the molybdenum ribbon, as shown in FIG. 4, while being insufiicient to cause recrystallization in the tungsten rod. Nor was the pressure sufficient to extrude or crack the molybdenum ribbon. Prior to closure of the slot there was poor heat transfer between rod 7 and ribbon 5, so ribbon 5 was not heated to the same extent as was rod 7.

Throughout the process, the assembly was blanketed with forming gas percent hydrogen, 85 percent nitrogen) in order to prevent oxidation of any of the metal.

The more uniform joint of the present invention, as contrasted with a prior art brazed connection where there are irregular surfaces between the rod and the ribbon results in several unexpected advantages. First, the stresses exerted on the glass by the connection are reduced. Second, the speed of evacuation of the lamp envelope, during the exhausting and filling process, is increased.

FIG. 2 shows an example of a tungsten halogen lamp utilizing a seal in accordance with the present invention. The lamp has a high silica glass envelope 1 1 having a press seal 12 at one end thereof, a coiled tungsten filament 13 disposed within envelope l1 and a lamp fill including halogen. Filament 13 has a coiled leg 14 at each end thereof, both legs 14 extending into press seal 12. Each coiled leg 14 has a tungsten rod 16 securely inserted therein, the end of rod 16 protruding slightly beyond the end of leg 14 and being slotted.

Embedded within press seal 12 are two spaced apart molybdenum ribbons 15. Each ribbon 15 is fastened within the slot of a rod 16 in the manner described above. An external lead-in wire 18 is connected to each ribbon 15 in order to provide for connection to an external power source.

It may be desirable, in order to minimize electrical contact resistance between a slotted rod and the metal ribbon, to fold the end of the ribbon upon itself and insert the folded end into the slot prior to closure as shown in FIG. 5. The length of the folded rtion would only have to be about e ual to the de th of the s ot. Also, the slot would have to be wi ened somew at to permit insertion of the folded end. However, the slot should not be so wide as to result in fracture of the metal when the sides of the slot are pressed and closed.

I claim:

. l. A lamp comprising: a high silica glass envelope having a press seal; a thin refractory metal ribbon entirely embedded within said press sea; a rod embedded within said press seal and extending interiorly into said envelope, said rod having a slot at the embedded end thereof, the sides of said slot being physically deformed in order to be securely closed on one end of said ribbon, thereby establishing a low resistance electrical connection between said ribbon and said rod.

2. The lamp of claim 1 wherein said ribbon is made of molybdenum.

3. The lamp of claim 1 wherein said rod is made of tungsten.

4. The lamp of claim 1 wherein said rod is connected to an arc discharge electrode.

5. The lamp of claim 1 wherein said rod is connected to a tungsten filament.

6. The lamp of claim 1 wherein said one end of said ribbon is folded upon itself. 

2. The lamp of claim 1 wherein said ribbon is made of molybdenum.
 3. The lamp of claim 1 wherein said rod is made of tungsten.
 4. The lamp of claim 1 wherein said rod is connected to an arc discharge electrode.
 5. The lamp of claim 1 wherein said rod is connected to a tungsten filament.
 6. The lamp of claim 1 wherein said one end of said ribbon is folded upon itself. 